Standard Model Introduction. Quarknet Syracuse Summer Institute Particle Physics

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1 Standard Model Introdtion 1 Qarknet Syrase Smmer Institte Partile Physis

2 Letre 2 Topis for Letre 2 Introdtion to Standard Model Eletromagneti & Strong Interations 2

3 Prelde Definition of Theory: a oherent grop of tested general propositions, ommonly regarded as orret, that an be sed as priniples of explanation and predition for a lass of phenomena. Important to add & reall: Theory is something that is falsifiable, bt not provable. Theories we teah abot have proven to be preditive, and therefore we believe them bt they are still falsifiable. Even Deeper Theory Deeper Theory Theory When do yo know that yo ve reahed the ltimate theory The Theory of Everything!? 3

4 The Standard Model Partiles Aim of researh into matter is to ome p with the most fndamental desription of partiles and fores years have lead s to a fairly eonomial pitre. I II III All fndamental partiles to date are fond to be fermions (spin 1/2) of type: Qarks (q=+2/3[,,t] or 1/3[d,s,b]) Leptons (q=0 or 1) Antipartiles have opposite harge and same mass as these partiles. 4

5 The Standard Model Fores Partiles interat via fores. What is fore? Yeah, OK, a psh or a pll F = dp/dt, an exhange of momentm. The modern view of fores is that they reslt from an interation between two partiles Not srprising, even Newton s 3 rd Law tells s that! In the modern view, the interation is proded by the exhange of a fore arrier. Eah fore has one (or more) arriers that mediate the interation in qestion. The photon is the mediator of the eletromagneti interation. 5

6 The Hydrogen atom Yo all know that the prinipal energy levels of the H atom is given by the simple formla: E n = 13.6 ev / n 2 Bt where does the 13.6 ev ome from? From qantm physis, yo an show that: 13.6 ev 2 m e e 2 2 m e e em( m ) e The qantity em is alled the EM opling onstant. It s vale is ~1/137, and gives the strength of the opling between a harge q=±e and a photon. What wold em be the opling between a and a p qark? 6

7 What is harge anyway? What is eletri harge? I really don t know, do yo? Best answer I have: It is an intrinsi property that allows ertain partiles to interat via the EM fore, i.e. interat with photons. No harge no play with Dr. photon! The strength of the interation will depend on the harge (larger harge larger fore) What is mass? What is spin? 7

8 Copling onstants In the H atom, the eletron is bond de to the ontinal exhange of photons between the eletron and the proton. The strength of that interation is diretly proportional to the EM opling onstant, em! In general, all fore arriers will ople to partiles with some strength. The partile in qestion mst have the right harge (more on this as we go) The larger the opling onstant, the large the energy level splittings, larger probability of interation orring, et. 8

9 The EM fore The qantm desription of EM interations of harged partiles is alled QED (Qantm EletroDynamis). Rihard Feynman was a pioneer in developing QED. Thanks to him (and others), we an draw diagrams of interations, apply well known Feynman rles to them, and allate the rate or probability of some proess. So alled Feynman diagrams We won t allate anything, bt they are a very sefl visal aid 9

10 Bak to fores Why does a nles with many (repelling) protons stay together? The protons are so lose, the replsive EM fore mst be ginormos! From Newtonian physis, yo know there MUST be a STRONGER fore that s holding them all together! The STRONG FORCE 10

11 Strong Fore The strongest of the for fores responsible for holding the nles together. Modeled after QED, de its spetalar sess. It is alled QCD (QantmChromoDynamis) QCD is really a theory of the interations of qarks The binding between protons and netrons is a residal effet of the QCD interations. Ex: Hydrogen, van der Waal, ovalent, ioni, et bonds in hemistry are NOT fndamental fores. They re all residal effets of the EM fore! QCD has some similar featres to QED A fore arrier (glon, also massless) It oples to some intrinsi property of qarks (olor harge) Big differene: range limited to ~nlear sizes ( range for EM fore ) 11

12 Hadrons We find in natre that qarks are bond p in two possible ways: Baryons: 3 qarks stk together proton = (d), netron = (dd) Mesons: qark + antiqark bond together pions: + = (d), = (d), 0 = (1/ 2)(+dd) Reall, in eletromagnetism, it is the eletri harge that allows them to interat via the EM fore (photons) Qarks arry olor harge, and it is this property (not eletri harge) that allows them to interat via the strong fore (glons) Bt instead of (+, ), qarks have 3 possible vales of this attribte. For historial reasons, they re labeled RED, GREEN & BLUE. 12

13 Matter Matter in the Standard Model Qarks Leptons t t t t t t e e + d d d s s s b b b d d d s s s b b b e e Not observed freely in natre? Observed freely : Atoms, Nleosynthesis olor harge d leads to qarks binding together proton netron Baryons b + B 0 Mesons Hydrogen 13